In this courtroom sketch, Faisal Shahzad pleads guilty to 10 terrorism and weapons counts June 21, 2010, in Manhattan Federal Court involving the failed car bombing in New York's Times Square. Shahzad was sentenced to life in prison.

Elizabeth Williams/AP

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In the years since the terrorist attacks of 9/11, the number of Muslim-American terrorism suspects and perpetrators has averaged about 16 per year. In 2010, according to a new report, the total was 20.

But 2009 likely was an aberration – the year when a group of 17 Somali-Americans joined Al Shabab, the Islamist insurgent movement linked to Al Qaeda. The number of individual Muslim-Americans plotting against targets in the United States also dropped by half, from 18 in 2009 to 10 in 2010.

“Of course, even a single terrorist plot is too many,” says Charles Kurzman, professor of sociology at the University of North Carolina at Chapel Hill and the report’s author. “But this trend offers a challenge for the American public: If we ratchet up our security concerns when the rate of terrorism rises, should we ratchet down our concerns when it falls?”

That is certainly not the case for federal, state, and local law enforcement authorities tasked with preventing domestic terrorist attacks.

While most attacks have been disrupted or failed on their own, 11 attacks since 9/11 have resulted in 33 deaths – including 13 people killed by Nidal Hasan at Fort Hood, Tex., in 2009. The Times Square bombing attempt by Pakistan-born Faisal Shahzad could have brought the total deaths due to domestic terror attacks much higher if the bomb had not failed to explode when ignited.

List of 2010 plots

According to professor Mr. Kurzman’s analysis, 75 percent of the Muslim-Americans engaged in terrorist plots in 2010 were disrupted in an early stage of planning.

“This is consistent with the pattern of disruption since 9/11,” he writes, when 102 of 161 plots – 63 percent – were disrupted at an early stage of planning.

Over the past five years or so, about 30 American Muslim extremists have been caught up in sting operations, according to Mark Pitcavage, director of investigative research at the Anti-Defamation League.

Most recently, that includes Antonio Martinez (a Muslim convert who had changed his name to Muhammad Hussain), who attempted to detonate a car bomb at a US Army recruitment center in Maryland, and Somalia-born Mohamed Osman Mohamud, arrested in December for allegedly plotting to explode a bomb at the Pioneer Courthouse Square in Portland, Ore., where thousands of families had gathered for the traditional Christmas tree lighting.

“Many of these extremists have a passionate desire to act on behalf of their cause, but in a practical sense have a limited ability,” says Mr. Pitcavage. “Part of this is due to their selection of tactics, which often tend towards large and spectacular attacks that are difficult to carry out.”

“Part of this is due to an inability on their part to obtain or construct weapons or explosives on their own,” he writes in an e-mail. “Thus when they encounter someone purporting to be able to supply such resources [including undercover FBI agents], they may well be receptive.”

Targeting the 'lone wolf'

It’s often pointed out that security agencies need to prevent all attempted attacks in order to be successful but that terrorists need to succeed just once. Of particular concern are home-grown “lone wolf” attackers, seen as expendable to terrorist groups overseas.

For this reason, sting operations frequently are the technique of choice in heading off such attacks.

“It does send a message that the government isn’t just leaving the barn door open like they were before 9/11, that it will have some kind of either limiting or deterrent impact on those who don’t come under the scrutiny of authority,” says Brian Levin, director of the Center for the Study of Hate & Extremism at California State University, San Bernardino.

As Kurzman at the University of North Carolina points out, sting operations often begin with tips from the Muslim-American community itself – the largest single source of initial information, according to his research.